This invention relates to a method and circuitry for controlling the intermediate frequency of an FM receiver such that the phase distortion of the FM receiver in the intermediate frequency amplification stage is minimized during broadcast reception.
Distortion caused by band-pass filters in the intermediate frequency amplification stage comprises the majority of FM receiver distortions. The distortion caused by band-pass filters is highly frequency-dependent and increases suddenly as the intermediate frequency deviates from the optimal value. Therefore, a method has been proposed in U.S. Pat. No. 4,237,556 to solve this problem by modulating the intermediate frequency signals at a specific frequency, detecting the even number distortions from the detection output, and controlling the intermediate frequency by using the phase so that the specific component in the distortion becomes zero, thus resulting in almost minimum distortion at the intermediate frequency amplification stage.
However, when the above-mentioned method is used, aside from the shortcoming that the complex construction may be costly, there appears to be another shortcoming in that the distortion is not always controlled at the minimum point due to the focusing on a specific component alone in the distortion.